This invention relates to polypropylene resins, more particularly to improved compositions comprising a crystalline polypropylene and still more particularly to blends comprising polypropylene and polyphthalamide having improved tensile properties and rigidity, and to methods for improving the rigidity of crystalline polypropylene resins. The improved blends have excellent thermal processability, and provide thermally processable filled compositions and composites.
The highly desirable balance of mechanical and thermal property characteristics of crystalline polypropylene has gained these resins wide acceptance in the molding and extrusion art. The resins are found in a great variety of applications, and may be particularly attractive for use where resistance to chemical and environmental attack is required. However, unmodified polypropylenes are generally low in impact. Further, though the flexibility of these resins makes them highly desirable for many uses, for application where a greater rigidity is desired reinforcement, for example with glass fiber or the like, may be necessary to provide the needed increase in rigidity.
considerable effort has been made by the resin arts over the years to develop methods for improving the mechanical properties of crystalline polypropylene. A great variety of modifiers and additives that may be found useful for overcoming one or more deficiencies has resulted from these efforts, though improvement in a single resin property is quite often accomplished only at some sacrifice in the overall balance of properties. For example, although addition of a variety of copolyolefin rubbers or polyethylene resins to polyolefins may usefully increase impact resistance, in most instances such additives also effect a lowering of rigidity and heat resistance. Similarly, polyamides have been added to polyolefins to improve oil resistance and tensile properties, but the incompatibility of unmodified polyamides and polyolefins may cause the resulting blends to be generally poor in important mechanical properties, and may even cause partial or complete loss of integrity for articles molded or extruded from such blends. The incompatibility of polyolefins with polyamides has been overcome in the art by modifying the polyolefin component to obtain a stable dispersion of the polyamide in the polypropylene. See U.S. Pat. No. 4,988,764.
Polypropylene resins may also be modified with fiber, and glass fiber-reinforced polypropylene resins are particularly well known and widely sold for uses requiring increased rigidity and improved dimensional stability, particularly at elevated temperatures. The addition of glass fiber to polypropylene generally provides little improvement in other mechanical properties, largely because of poor adhesion to the glass fiber surface. Modification of the polypropylene with unsaturated carboxylic acid compounds and treatment of the fiber with coupling agents to provide a chemical interaction between the modified polypropylene and the fiber surfaces are among the methods that have been employed for overcoming these deficiencies, though with varying degrees of success. Such modifications frequently reduce the attractiveness and visual appearance of the surfaces of molded articles, making further efforts necessary to overcome these added problems. For example, in U.S. Pat. No. 4,613,647 there is described a method for improving the compatibility of glass reinforced, carboxyl-modified polypropylene by the addition of polyamides such as nylon 6 or nylon 66. However, the improved compositions are said to suffer because of an inherent reactivity of the polyamide with the carboxyl-modified polypropylene, resulting in yellowing and delustering in molded products. The patent further discloses overcoming these added problems through the use of meta-xylylene adipamides as the polyamide component of such compositions; even with these modifications, the resulting glass-reinforced blends remain somewhat deficient in rigidity and tensile properties.
Improved polypropylene resin formulations, both filled and neat, that are improved over polypropylene in rigidity and thermal characteristics without substantially reducing thermal processability, particularly moldability, are continually sought by the art for use in the production of molded and extruded articles. Such compositions could find wide acceptance in the plastic art, and such compositions would be particularly attractive for use in demanding environments where elevated temperatures and corrosive conditions may be encountered.